Fix skipped Celery test in backend

[alehuo]

Describe the bug

Celery test is skipped in backend because of a DB transaction bug.

[saimouu]

The rest of the tests just rollback by default at the end, whereas celery handles the transactions by itself and commit between db operations. Meaning that the db is not in clean state for other tests.

Not sure what is the default way to handle this. Maybe running tests that deal with celery separately (or at the end) and just looking the results with the given id, rather than querying all?

[alehuo]

One option would be to just mock celery and think about meaningful test cases.